28 Chapter 1 ■ Introduction
image database containing high-resolution satellite images of a region in con- nection with change-detection applications. In addition to guiding the operation of each processing module, the knowledge base also controls the interaction between modules. This distinction is made in Fig. 1.23 by the use of double- headed arrows between the processing modules and the knowledge base, as op- posed to single-headed arrows linking the processing modules.
Although we do not discuss image display explicitly at this point, it is impor- tant to keep in mind that viewing the results of image processing can take place at the output of any stage in Fig. 1.23. We also note that not all image processing applications require the complexity of interactions implied by Fig. 1.23. In fact, not even all those modules are needed in some cases. For example, image enhance- ment for human visual interpretation seldom requires use of any of the other stages in Fig. 1.23. In general, however, as the complexity of an image processing task increases, so does the number of processes required to solve the problem.
1.5 ■ Components of an Image Processing System 29
Image displays Computer Mass storage
Hardcopy
Specialized image processing hardware
Image sensors
Problem domain
Image processing software
Network FIGURE 1.24
Components of a general-purpose image processing system.
distinguishing characteristic is speed. In other words, this unit performs functions that require fast data throughputs (e.g., digitizing and averaging video images at 30 framess) that the typical main computer cannot handle.
The computerin an image processing system is a general-purpose computer and can range from a PC to a supercomputer. In dedicated applications, some- times specially designed computers are used to achieve a required level of per- formance, but our interest here is on general-purpose image processing systems.
In these systems, almost any well-equipped PC-type machine is suitable for off- line image processing tasks.
Softwarefor image processing consists of specialized modules that perform specific tasks. A well-designed package also includes the capability for the user to write code that, as a minimum, utilizes the specialized modules. More so- phisticated software packages allow the integration of those modules and gen- eral-purpose software commands from at least one computer language.
Mass storagecapability is a must in image processing applications. An image of size 1024*1024pixels, in which the intensity of each pixel is an 8-bit quan- tity, requires one megabyte of storage space if the image is not compressed.
When dealing with thousands, or even millions, of images, providing adequate storage in an image processing system can be a challenge. Digital storage for
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30 Chapter 1 ■ Introduction
image processing applications falls into three principal categories: (1) short- term storage for use during processing, (2) on-line storage for relatively fast re- call, and (3) archival storage, characterized by infrequent access. Storage is measured in bytes (eight bits), Kbytes (one thousand bytes), Mbytes (one mil- lion bytes), Gbytes (meaning giga, or one billion, bytes), and Tbytes (meaning tera, or one trillion, bytes).
One method of providing short-term storage is computer memory. Another is by specialized boards, called frame buffers, that store one or more images and can be accessed rapidly, usually at video rates (e.g., at 30 complete images per second). The latter method allows virtually instantaneous image zoom, as well as scroll(vertical shifts) and pan(horizontal shifts). Frame buffers usually are housed in the specialized image processing hardware unit shown in Fig. 1.24. On- line storage generally takes the form of magnetic disks or optical-media stor- age.The key factor characterizing on-line storage is frequent access to the stored data. Finally, archival storage is characterized by massive storage requirements but infrequent need for access. Magnetic tapes and optical disks housed in
“jukeboxes” are the usual media for archival applications.
Image displaysin use today are mainly color (preferably flat screen) TV mon- itors. Monitors are driven by the outputs of image and graphics display cards that are an integral part of the computer system. Seldom are there requirements for image display applications that cannot be met by display cards available com- mercially as part of the computer system. In some cases, it is necessary to have stereo displays, and these are implemented in the form of headgear containing two small displays embedded in goggles worn by the user.
Hardcopydevices for recording images include laser printers, film cam- eras, heat-sensitive devices, inkjet units, and digital units, such as optical and CD-ROM disks. Film provides the highest possible resolution, but paper is the obvious medium of choice for written material. For presentations, images are dis- played on film transparencies or in a digital medium if image projection equip- ment is used.The latter approach is gaining acceptance as the standard for image presentations.
Networkingis almost a default function in any computer system in use today.
Because of the large amount of data inherent in image processing applications, the key consideration in image transmission is bandwidth. In dedicated net- works, this typically is not a problem, but communications with remote sites via the Internet are not always as efficient. Fortunately, this situation is improving quickly as a result of optical fiber and other broadband technologies.
Summary
The main purpose of the material presented in this chapter is to provide a sense of per- spective about the origins of digital image processing and, more important, about cur- rent and future areas of application of this technology. Although the coverage of these topics in this chapter was necessarily incomplete due to space limitations, it should have left the reader with a clear impression of the breadth and practical scope of digital image processing. As we proceed in the following chapters with the development of image pro- cessing theory and applications, numerous examples are provided to keep a clear focus GONZ01-001-033.II 29-08-2001 14:42 Page 30